Tuning indicator circuit



Oct. 29, I940. K, sTEiMEL ET AL 2,219,505

TUNING INDICATOR CIRCUIT Filed Oct. 8. 19s? a ssrre-ets-shet '1 INVENTORS I64RL STE/MEL KUR 5 BY 'wzw Oct. 29, 1940. K. STEIMEL El A'L TUNING INDICATOR CIRCUIT 5 Sheets-Sheiet 3.

'Filed Oct. 8, 1937 7'0 PRIOR TUBE ANODE MFR/0R TUBEANUDE 70 PR/m? TUBE AA/ODE MMMIRUI. GRID 0F PRIHRM'BE I mE/vp was" PRMR 77/85 o INVENTORS KARL STE/MEL BY KURT IE A. o'RNEY Oct. 29, 1940. K. STEIMEL El AL.

TUNING INDICATOR CIRCUIT 5 Sheets-Sheet 5 Filed 001;. 8, 1937 QPR/OR TUBE CONT'ROL GRID r0 PRIOR TUBEANUDE MAE-AMPLIFIER vvv 7 0 PRIOR rum. CONTROL GRID INVENTORS KARLSTEIMEL KURT M/ AITORNEY r Patented Oct. 29, 1940 UNITED STATES PATENT o FmE Karl Steimel and Kurt Mie, Berlin, Germany,'as signers, by mesne assignments, to Telefunken Gesellschaft fur Drahtlose Telegraphic m; b; H;, Berlin, Germany, a corporation of Germany 1 Application October 8, 1937, Serial No. 167,977 g In Germany October 8, 1936- 1' Claim.

, :Often it is customary to render visible the presence of voltage by optical means. These means may consist, for instance, of a glowdisf charge tubewhich becomes ignited only when the voltages exceed a certain limit. It is already known 'to' use fluorescent electrodes for such optical voltage indicators, said electrodes radiating 'light when impinged upon by electrons, and furnishing a luminous pattern varyingas to position or shape. A known' embodiment of this principle of indication utilizes a directly'heated glow cathode extending in one direction, furthermore a plate-shaped control anode likewise arranged parallel-to said cathode and placed in back of the grid. The voltage drop through the directly heated cathode produces in the longitudinal direction of, the system continuously varying field conditions. A negatively biased grid blocks, according to the value of its biasing potential, the entire electron stream passextending in the same direction is necessary at such an arrangement requires a complicated cathode arrangement for the production of the v concentrated electron ray thus involving high cost. Furthermore, in many cases it is not advisable to render the voltage recognizable simply by the change in position of the indicating light spot.

In a still other indicating arrangement an indirectly heated cathode is employed so that this arrangement can also be used for alternating cur- F rent. In the preferred embodiment this cathode electrode in parallel thereto and a fluorescent is surrounded coaxially by a cylindrical or coni- I employed in radio apparatus are insufficient'and j must first be increased. Therefore, in the said arrangement, in the same bulb also an amplification system of ordinary construction is arranged 1 below the same cathode, and by means of which the control potentials are amplified for the neces- 1 sary'values, This additional amplifying system obviously increases the cost of such an indicating arrangement. However,v the amplifying system may in this case, and to a c'ertainextent also be used at the same time for other purposes. Ordinarily, in the above-described arrange ment the anode of theam'plification'part is ex tended into the indicating part for instance by extending" the anode supporting wires into the other part But suggestions have alsobeen made :2 to extendthe control electrode of the amplifying system into the indicating system. Tests have proven that :also such arrangement reveals a rather low sensitivity, and requires high control potentials. The comparatively low sensitivity of the above-describedarrangement is due to the fact that the electrons already have arvery high velocity'at-the place of deviation. The control electrodes shaped as pins, or sharp edges, in fact have but a small effect upon the field intensity 3 of the cathode so that the, electron emission is substantially determined by the plate potential. Consequently,'the electrons undergo immediate acceleration after having been emitted, and thus they have already: a considerable velocity in the 40 vicinity ofthe control electrodes.

On the basis of the teaching that the field intensity as wellas the-potential must be maintainedlow at the place of control so as to maintain low the velocity of the electrons to be controlled, the present invention provides means by which this canbe accomplished. In accordance with the present invention, in an optical indicating arrangement for voltage, in which an electrode which becomes fluorescent by the action of impinging electrons, and the voltage tobe measlured'isflindicated by the extent of the angular range on which light appears, the control element is so designed, orsuch'additional means are provided, that akcontrol space is formed which surv rounds the cathode, and the potential and field intensity are decreased at the place of control.

This end is attained in the simplest manner by means of a grid surrounding the cathode. Such grid shields the cathode against the anode so that only a portion of the plate potential remains directly active on the cathode, while the field intensity on the cathode is substantially determined by the potential of the grid. Thus, if a potential is impressed upon the grid which in the average is far below the plate potential, for instance cathode potential or a negative potential, the electrons emitted from the cathode move with comparatively low velocity through the path leading to the grid, and only after arriving in back of the grid are they accelerated to the point at which they reach the velocity necessary for exciting fluorescence. It was found that the control potentials of the order of a few volts'as normally used in radio apparatus for the fading control or the like, are sufiicient to cover the entire angular range.

At the same time such a grid surrounding the cathode oiiers the possibility that aside from the control of the angular range the intensity can be controlled. Thus, there is possible the following indication system which is very favorable from a. psychological point of view: Without voltage the optical voltage indicator remains dark, at low voltages it shows a brightening up, and at maximum voltage there results a sharply limited luminous surface whose angular range varies in dependence on the value of the indicated voltage. Such simultaneous control of the angle and intensity is not possible with the known arrangements and ofiers a particular advantage of the arrangement according to the present invention.

Furthermore, the diminution of the potential near the cathode by the introduction of the grid has the following, technically very important advantage.

Toexcite the fluorescent coating of the plate so that it emits visible light, requires relatively high voltages, but only small currents. If the currents are increased, the life of the coating is reduced. Therefore, it is necessary to have the currents limited.

Without a grid near the cathode the current can be limited only by the saturation of the cathode. This method of limiting the current however has essential disadvantages.

Difficulties already arise during activating the cathode for the coating and the cathode may be deteriorated by the heavy currents or by occasional outbursts of gas during the process.

Images of non-emitting spots, which happen to be on the cathode, will be clearly visibly projected on the plate, disturbing the pattern of light thereon.

Finally a saturation limited current is highly dependent on the temperature of the cathode. so changes in filament voltage, which cannot be avoided in practice, will produce similar changes in plate current and brightness of the light pattern, which are very troublesome in certain circuit connections.

By introducing the grid these disadvantages are completely avoided, because in this case the current is limited by space charge, which can build up in consequence of the said diminution of the potential near the cathode by the grid.

These advantages achieved by the introduction of the grid during the activation of the cathode and during the life of the device are so important, that they warrant the introduction of the grid even if this grid is not to be used to control the current, but is held on a fixed potential such as the cathode potential.

Since it is desirable that at the last control of the angle a constant density of the light be maintained, it will be of advantage if the control elements determining the angle are removed as far as possible from the cathode. Thus the control elements shaped as pins, or sharp edges, will have to be arranged in the vicinity of the grid electrode either inside or outside the same. It is even possible'to have the control pins at the place of the grid supports so that practically the grid acts simultaneously as a shield and as a control element. The shielding action is taken over by the grid winding, while the angle control is exerted by the grid bridges.

For a better understanding of the invention reference will now be made to the accompanying drawings wherein Figs. 1a, 2a and 3a illustrate various constructions of the visual indicator portion of electron tubes according to the present invention. Figs. lb, 21) and 3b are the respective plan views of Figs. 1a, 2a. and 3a. Figs. 4, 5 and 6 represent partial circuits which illustrate various connections of the electrodes constituting the visual indicator tubes, and Figs. 7 to 16 illustrate more complete circuits showing further arrangements according to the invention.

In Fig. 1a, I designates the glow cathode, 2 represents the pin-shaped control electrodes, 3 is the grid acting as a screen, 4 is the fluorescent anode having the known conical shape and supported by the members-i. The entire system is supported by an insulating disk 6 inserted into the tube 1. In Fig. 1b the lines 8 represent the angular range illuminated by the impinging electrons on the anode. Fig. 2a difiers from Fig. 1a only by the omission of special controlling pins, the grid supports 3 taking over the control of the angular range. These supports may eventually be thicker than is required. In both figures in place of the pin-shaped control bridges as shown, edge-shaped or band-shaped control members may be employed. The conical anode may, also, be replaced by one having a cylindrical, or similar, shape.

For indicating by light, only a small electron current is necessary. The required energy is so low, and the dimensions of the indicating system can be so small, that a voltage indicator according to the invention can be combined without difficulty with an ordinary amplifying system for radio purposes, or the like. The combination, according to a further feature of the invention, of a voltage indicator with one or several ordinary discharge systems represents a reduction in cost as compared with the above-described known arrangement, because the other systems combined with the indicating section are available for other purposes also. Hence, the indicating section does not require a special bulb.

Fig. 3a shows such a construction combined with an ordinary triode. The reference characters for the voltage indicator are the same as in the previous figures. In the amplifying section, 9 represents the anode, and I0 is the control grid. As seen from the figure, in a manner similar to the known arrangement, the anode supports 2 are extended into the indicating section, and also serve for controlling the angular range. They are arranged outside the control grid, contrary to Fig. 1a wherein the control pins 2 are inside the grid.

In order optically to shield the glow cathode "thereby facilitating the viewing, the grid is suit ablyclosed up bya cap H at the end pointing towards the place of viewing. Such cap, or similar light stop, may as is obvious also be provided in'the arrangement according to Fig. 1a or 211. Since the cap is fixedly secured'to the grid, and the grid has in general a low potential, danger exists that the cap will cause too high-a decrease ofthe electron stream passing to the anode, thus obtaining only a narrow luminous stripe. In order to prevent this condition the grid can be extended in this direction to such an extent beyond the system that the field caused by the cap does not produce a detrimental efiect. v

In placeof the triode systemv shown, obviously any other system may be used. Furthermore, it isnot necessary to provide the control electrodes in pairs, only a single controlling Din may be provided instead. On the contrary a greater 'numberof pins may be arranged. If the indicating section is combined with an amplifying system containing more than three electrodes,

then any other electrode may be extended as a control element into the indicating space. In all cases, in place of the anode the control grid may be used for the additional control of the angle.

.As already pointed out above, the amplifying system is absolutely independent of the indication section and'may be utilized for any other purpose. This is also true if one of the amplifier electrodes protrudes into the-indicating section.

In such a case it is always possible'soxto arrange the other electrodes, especially the grid, that the additional control exerted by the extended elece T trode amounts to only a small part, while the main control is effected for instance by the bridges of the grid. This independence of the f two systems offersa multitude of possibilities of "4o connections representing a special characteristic 7 feature of the arrangement according to the invention.

: The combination of circuits resulting from. the

' combining of the two systems can be grouped into four'different cases. Infthe case I, the amplifying section is connected as a resistance amamplifier for the control potential, and directly controlled by the anode extensions protruding into the indicating section, while the gridserving as a screen may be maintained at afixed potential such as by connecting it directly with the I cathode, or else it may have applied thereto a variable potential which may be manually controlled. This case is similar to the circuit used in the last of the known arrangements herein mentioned. In the case II, the two'systems are independent of each other except for the lastmentioned limitation. The control of the indication section is carried out solely by the grid arranged within said part, and which for .the

sake of convenience shall be termed a regulation grid. To this end, the grid has applied from any suitable place of the apparatus a control potential having the proper sense. This place may be a precedingstage, or a subsequent stage,of the apparatus, or the stage proper which is combined with the indicating system. Contrary to case I, in this case, however, the control potential for the regulation grid is tapped from 'a resistor in the circuit of this stage, so that it will then also be available for other purposes. The case III represents the combination of case I and II. It can bedivided into the two subcases namely that (a) the two controls act in the same sense, thus supporting eachlother, and

' the preliminary stage.

that (b) the two controls are opposite whereby the 'control corresponding to the case I must have a muchsmaller influence than the control according tocase II- The case IV resides in that the indicating section, having the appearance of a triode, is used at the same time for any other purposes.

In the circuit diagrams, the individual cases are elucidated by individual special examples.

Figure 4. corresponds to the case I. The control grid I of the amplifier section, represented as I y a triode, has. applied thereto the control potential produced by a diode path, or by an audion.

The anode 2' of and whichis amplified thereat. this section has extensions 3" in the indication part throughwhich an angle control is carried out in accordance with a change of the control potential. The regulation grid 4 acting in this case as a screen is directly connected to the tion' grid 4 'is tapped at the cathode resistor. which, in fact, carries the current passing inthe amplifying part. In the example shown the control potential is produced by a diode path. The remaining circuit corresponds with the ordinary arrangements. For the sake of elucida-" tion it should simply'be borne in mind that the resistor R is connected with a positive voltage source. i

In the arrangement according to Fig. 7, the

control of the tuning indicator tube M is used simultaneously according to Figs. 4 and 5. The

radio or intermediate frequency is fed from the output of the preceding tube through the band pass filter Bf to the left plate of diode D. Said diode produces the control potential for the preceding tube through resistance R7. Simultaneously over resistance R10 audio frequency is derived from resistance. R7 by way of the e1e-- 'ments R11, C4 and the volume control resistance R5 which is A. C. grounded over condenser C5.

A variable portion of the audio frequency voltage developed across R5 is amplified by the audio frequency tube N which is resistance-condenser coupled to the finalstage in the usual manner. Thegrid l of the tuning indicator tube M is connected to the control potential, for the preceding tubes over a filter element consisting of resistance Rs and condenser C1. The necessary grid bias is applied to said grid simultaneously with the audio frequency tube N through the denser C3. Resistance Rs furthermore effects a retarding of the control potential sothat said potential and therefore also the indication does not begin before a given radio or intermediate frequency amplitude. is attained. Due to the connection of the grid l of the triode system ofthe tuning indicator tube M to the control potential, the potential of the' control grid becomes more negative with increasing radio or cathode resistances R3 and R9 bridged by con- I a intermediate frequency of the diode D. As a result the plate current becomes smaller and the plate potential increases due to the smaller voltage drop across the plate resistance R1. This effects a broadening of the light angle through the light-control electrodes 3". With increasing plate potential the potential of the regulating grid 4 taken off between resistances R2, R3 also increases, whereby again a broadening of the light angle is brought about. Both controls over the control grid I, on the one hand, and over the regulating grid 4, on the other hand, assist one another, and thus eifect a high sensitivity beginning with a light angle of up to about 160. In addition to its function as a tuning indicator the tube M has a second purposein the circuit just described. A sufliciently high negative potential is applied from the potential source Sp through resistance R4 to the grid of the audio frequency tube N so that said tube is blocked. When radio or intermediate frequency is received the potential between resistances R2 and R3 becomes more positive as mentioned above, and thus also the potential between resistances R and R4. At the instance where the negative bias of the regulating grid l becomes positive, grid current begins to flow whereby the high negative potential of the grid of the tube N is compensated and the blocking of said tube is terminated. As an aid the righthand discharge path of the diode D is connected in parallel to the regulating grid.

Fig. 8 shows an arrangement somewhat similar to Fig. 7. In this case also tuning indicator tube M has two functions, namely, indication and removal of blocking of the control potential for silent tuning. Instead of the battery Sp of Fig. 7 a different cathode level of the tuning indicator tube and of the diode is used. In this case a multiple purpose tube D, (diode and triode) is employed in place of the tubes D and N in Fig. '7. Said level is produced by the biasing resistance R9 which is bridged by condenser C and is possible by having the cathode currents of multiple purpose tube D and of tuning indication tube M pass through said resistance R9. At the same time, said level is applied to retardation of the control potential which is produced over resistance R8 and which is fed to the preceding tube over resistance R1. The second or right-hand diode path is connected to the cathode over resistance R10 and is fed, through condenser C4, the radio or intermediate frequency potential derived from the output of the band-pass filter. Said diode path serves for the production of the audio frequency potential and also for the production of the control potential for the tuning indicator tube M. As said control potential is not delayed or retarded the indication of the tuning indicator tube M begins with the weakest transmitter. The control potential is fed to the control grid I over a filter element consisting of resistance R6 and condenser C. The control of the regulating grid 4 is performed over the voltage divider consisting of resistances R1, R2, R3, to which the screen grids of the preceding tubes are connected between resistances R1 and R2. Between the resistances R4 and R5 which are connected in parallel to resistance R1 of the voltage divider, there is located the tap for the plate potential of the multiple purpose tube D and a variable tap on the resistance R5 is connected to the plate of indicator tube M. With signal reception the control grid of the tuning indicating tube becomes more negative due to the control potential produced by the right-hand diode path. As a result the current flowing in this portion decreases and opens the audio frequency portion of the multiple purpose tube D due to the lower potential drop at resistance R9 and due to the voltage increase at the top of the plate potential for the multiple purpose tube. Simultaneously a broadening of the light angle occurs corresponding to the signal. The control of the regulating grid 4 is performed also by the increase of the potential between resistances R2 and R3.

The audio frequency voltage is taken off the diode load resistance Rm over resistance R11 and condenser C10, and is fed to the volume control and compensated tone control network consisting of resistances R13 and R14 and condenser Cs. The slider is connected to the grid of the multiple purpose tube D through resistance R12. The amplified audio frequency potential is fed from the plate of the multiple purpose tube to the control grid of the final amplifier tube E through the usual resistance-condenser coupling network.

In the arrangement according to Fig. 9, the incoming signal is rectified in the left-hand diode path of the rectifier D and the control potential is produced across the load resistance R5, said control potential being fed to the control grid I of tuning indicator tube M through resistance R3. The control potential effects, at the plate 6 of the tuning indicator tube M the voltage of which is taken off the voltage divider R1, R2, an increase in potential and thereby a broadening of the light angle. The cathode of the tuning indicator tube M and of the diode D have a negative auxiliary potential (150 volts) over a common resistance R4 bridged by condenser C1. The voltage drop of this resistance has to be so adjusted that the cathodes possess a small positive potential with respect to ground. If the above-mentioned control occurs, the potential drop of resistance R4 decreases, that is the regulating grid 4 grounded over resistance Ra becomes more positive in regard to the cathode whereby, on the one hand, the indication is assisted, and, onthe other hand, the grid potential of the audio frequency tube N is driven over resistance R7, so that thereby the blocking of said tube is removed. The audio frequency voltage is taken ofi by means of the slider on the load resistance R5 and is fed to the grid of the audio frequency tube N through condenser C3.

Fig. 10 represents a modified arrangement of Fig.9. Instead of the diode D in the latter figure a multiple-purpose tube D is used and a separate audio frequency tube is thereby saved. The radio frequency signal produces at the combination R5C2, through the rectified current, the control potential which is fed through the control grid I of indicator tube M through resistance R3. When .a radio frequency signal is received the plate current of the tuning indicator tube M is decreased and thereby an increase in the potential across the voltage divider R1Rz will occur and also a broadening of the light angle. Tuning indicator tube M and multiple-purpose tube D have a common cathode resistance R6 at negative potential. Exactly as in the arrangement according to Fig. 9 the control effects simultaneously a decrease in the voltage drop across resistance Rs whereby the potential of theregulating grid lv becomes more tential of ,the point X on the voltage divider consisting of resistances R'1R9-R1o.. In thisv in.-

- stancethe right-hand diode path becomes operative and the blocking of the audio frequency tube is removed. Resistance R12 and condenser Cs effect the vusual filteringof the plate current for ;the multiple-purpose. tube at the external resistance R11 from whichthe amplified audio frequency potential is taken off through condenser (15., The taking off. of. the audio frequency upon rectification of the radio frequency in the diode pathis performed over the slider, of resistance R5, said slider being connected to the grid of the multiple-purpose tube through condenser C4, re-v s'istance Ra serving as the grid, leak for the controlgrid of :said tube. The control potential for the may be taken off the regulating grid 4' of the tuningindicator tube because when controlling R4, said voltage'drop being able to be used for control.

This control begins of course in a retarded' manner as the cathode is positive in regard to the, regulating grid before radio frequency signals are received.

Figflll representsa simplification of the arrangement shown in Fig. 10. A separate diode rectifier is here omitted and is replaced by the control'grid l' and the regulating grid 4' of the. tuning indicator tube M. Therefore, the output of band pass filter BI is directly connected to I the control grid oftuning indicator tube M and theaudio frequency voltage across load resistance R5 istaken off by way of resistance R4 and condenserC and fed to the grid of the audio I frequency tube N, the signal grid whereof is connected by way of resistance R6 to the regulating f grid 4' of the tuning indicator tube M. When radio frequency is received grid current fiows .between control grid 1 and cathode of' the tuning indicator tube so that a voltage drop occurs across resistance R5, said voltage drop negatively biasing the control grid of the tuning indicating tube M. This effects a decrease in plate current and therefore the usual increase in plate voltage and thus a broadening of the light angle. The

cathodes of the two tubes M, N are connected to a'negative auxiliary potential over a common resistance Re So that when regulating the regulating grid potential, said potential tends to assume more positive values and assist the indication.x In the instance where the grid current begins to flow through the regulating grid, the

blocking of the audio frequency tube N, the control grid of which is connected to the regulating 1 grid over resistance Re, Will be overcome. Si-

mu1taneously the voltage drop occurring across resistance Ps may serve for retarded control of the preceding tubes.

In the circuit of Fig. 12 there is shown a tuning indicator tube M functioning as a rectifier I .01 grid detection type. As in the circuit of the preceding figure, the decreasing'plate current caused by rectification effects a broadening of preceding, tubes the light; angle. W The amplified audio frequency is taken off the external resistance R1 over condenser vCzand is fed to the succeeding amplifier stages.- The cathode is connected to anegatlve auxiliary potential over resistance R3 so that acontrolpotential occurs, corresponding to the arrangement according to Fig. l1.

In the circuit according to Fig. 13 the tube preceding the indicator tube M is a hexode hav-; ing the grids I, 2, 3, 4, and acting as apentode. The radio or intermediate frequency is fed over a band pass filter Bj-to grid 1 and is taken off in amplified form at the plate circuit L1-C1 over condenser C2. It is then fed to the plate of the hexode. Between the grid '4 and the plate there is produced .a space charge which forms a virtual cathode for the plate. The plate thereforeoperates as diode plate and is connected accordingly. The control and audio frequency potential resulting developed across resistance R4 is fed to the control grid of indicator tube M through a filter network consisting of series resistances R5 and R6 and-shunt condenser C4. The voltage increase occurring through the control action across resistance R7 will again increase the light angle. The cathode of the tuning indicator tube M is at a negative auxiliary potential by way of the resistance Ra so that the voltage curve of the regulating grid described in Fig. 11 results and a retarded control potential is produced over resistances R9R1o, said control potential being fed, if desired to the preceding tubes. Resistances R1 and R2 form the voltage divider for applying potential to the screen grid 2 of the hexode while resist-ance R11 forms the grid leak resistance of the following tube, not shown. Th grid 3 is the suppressor and isconnected in the usual manner to the cathode.

Fig. 14 shows a pentode tube V. The screen gridvoltage divider consisting of resistances R1, R2, R3, and R4 controls the regulating grid 4 by way of resistance R7. The amplifier system of the tuning indicator -tubeM is used in this case for plate rectification. The audio frequency is taken off plate resistance R9' through condenser Cs and the control potential for the preceding tubes is produced across the voltage divider consisting of the resistances R5, R6, Re.

In this arrangement of the tuning indicator.

tube, contrary to the arrangements described above, the light angle is influenced through the control bridge in a direction opposite to those of the preceding arrangements. To be sure, in

a plate rectifier the plate currentincreases with increasing radio frequency potential and thus also the voltage drop over the external resistance R9 so that the plate potential decreases. This effects, as mentioned above, a narrowing of the light angle. The second control of the light angle occurring over the regulating grid effects however, due to its voltage curve, an increase of the light angle. As the last mentioned control influences the light angle much-more than the first one, its effect is overwhelming and the light angle will increase also when radio frequency is received, although with a somewhat decreased sensitivity.

In the arrangement according to Fig. 15, the radio frequency is fed to' the plates of diode D through a band pass filter Bf, the left-hand plate of said diode serving for demodulation in connection with resistance R1 and condenser C2.

The audio frequency is taken off the slider of resistance R1 and is fed to the grid of tube V through condenser C3. The second diode plate has radio frequency impressed thereon by Way of the con-denser C1 and serves for producing the control potential of the preceding tubes in connection with resistance R2. A selective tuning circuit S is coupled to the band pass filter Bf, said tuning circuit having its damping additionally removed by feedback. The control grid I of the indicator tube operates again as a diode plate. l/Vhen radio frequency is received indication sets in with very selective tuning of the receiver only, corresponding to the selectivity of tuning circuit S due to the voltage drop over resistance Rs bridged by condenser C8. The cathodes of all tubes are connected to a negative auxiliary potential over a common resistance R7, so that, in regard to the regulating grid 4' the same holds true as in the arrangement according to Fig. 11. i

In the arrangement according to Fig. 16, the intermediate frequency is fed through the selective tuning circuit S, the damping of which is additionally removed by feedback, to the control grid i of the tuning indicator tube M, said grid operating again as diode plate. The plate potential of said tube is derived from a voltage divider consisting of resistances R1, R2, R3, one end of which is connected to the negative auxiliary potential. The potential increase occurring over resistance R1 controls the light angle and also the regulating grid 4' which is connected to the third grid of a hexode. The grid current setting in with a given signal amplitude, (adjustable over resistance R1) varies the potential of resistance R3 and thereby removes the blocking of the intermediate frequency heXode N. The second band pass filter Bfz circuit controls in the usual manner a diode path producing a control potential over resistance R9, said potential being fed to the first grid of the hexode over a filter section consisting of resistance Rs, condenser C5 and resistance R7. Said potential may be branched ofi resistance R8 and used for the control of further tubes. The first grid of the hexode is coupled through condenser C8 to the second circuit B of the band pass filter Bh. The amplified intermediate frequency is taken 01f band pass filter Bfz at the plate side and is fed to the second or right hand path of the diode D. The audio frequency is taken off load resistor R10 and may be fed to further amplifier stages.

The above-described circuits show the various ways in which a combination tube according to the invention can be utilized. The enumeration is obviously without limit, and other circuits of different construction can be envisaged. In the description of the circuits, emphasis was laid solely on the especially interesting points in connection with the combination tube. The other circuit elements are those which are readily understood by anyone skilled in the art, and are shown to elucidate the manner in which the circuits according to the invention can be employed.

What is claimed is:

A circuit arrangement for the visual indication of voltages and for indicating the degree of tuning in radio apparatus, comprising an electron discharge tube containing an indicating section and an amplifying section, the latter including at least an anode, the indicating section comprising a cathode and a surrounding anode electrode rendered fluorescent by the action of the impinging electrons, a control electrode in the form of a rod positioned between the fluorescent anode and the cathode and having a connection to the amplifier anode, and a shield grid Within the cathode-anode space having a suitable potential impressed thereon to form a control space surrounding the cathode whereby the potential and the field intensity in the vicinity of the control electrode are decreased, and circuit means responsive to the received signal for applying a suitable potential on said shield grid for regulating the effect of said control space.

KARL S'IEMEL. KURT MIE. 

